Embodiments of the present invention relate to a controller for a SUDA system (also referred to as Shared User Equipment-Side Distributed Antenna System), to a method for controlling a SUDA system and to a computer program.
Already during their deployment, current 4G mobile communications systems (like LTE-Advanced) appear to suffer from a shortage of data rate that can be provided to the users. It is expected that in the future, the data rate requested by the users grows considerably, which is mainly due to reception of video contents. There is a trend to an increased consumption of non-linear TV/video, i.e. video contents that is not being broadcast at the very moment of its consumption. Besides broadcast contents that is consumed at some later point after its transmission (like the offering of the TV channels' media centers) and that could be stored inside a cache in the user equipment (UE) until its consumption, there is a vast realm of content that cannot simply be distributed by conventional broadcast systems (satellite, terrestrial, cable TV) like YouTube videos. At the same time the contents consumed in the homes necessitates increasingly high data rate, for instance for Ultra-High Definition TV (UHDTV) or 3D contents (with or without dedicated 3D-glasses).
Moreover, people exchange, i.e. down and upload increasingly large files. While this is currently photos of a couple of megabytes, people are going to download complete movies of many gigabytes from their mobile devices in the future. For such actions people are keen to keep the download times as short as possible, such that very high data rates in the order of ten gigabit/s are a realistic requirement for the future. As people are going to use cloud services to a greater extent in the future, there will be a need for fast synchronization of the contents on a mobile device with the cloud when people leave or enter the coverage of a mobile network, i.e. before they go off-line and after they return from on off-line state. The amount of data to synchronize could be quite large. All of this shows that transmission at very high data rates is a must in the future for many (mobile and stationary) devices.
An alternative to using mobile communications like LTE for downloading such large files is the employment of a local area network (LAN), be it wireless (WLAN, W-Fi) or wired (Ethernet). However, the last mile from the backbone network to the homes cannot support the necessitated high data rates in the range of Gbit/s, except if optical fibers are used (fiber-to-the-home FTTH). However, the cost to equip the homes with FTTH is very high; for instance for Germany alone, the cost to equip every building with FTTH is estimated around 93 billion/milliard Euros. Therefore, the last mile will eventually become a mainly wireless connection. This reduces the cost for bringing broadband to every building and its rooms significantly.
Moreover, most homes do not possess a dedicated wired LAN infrastructure (Ethernet) to distribute the data received over the last mile further, i.e. most homes employ Wi-Fi to connect their devices to the Internet by their access point (AP), where the AP represents the terminal point of the last mile. It should be observed that for reaching data rates of Gbit/s, either an Ethernet socket or an AP is necessitated in each room of every home or office building. Hence the cost of connecting each room of each building is added to the figure mentioned above for connecting the buildings.
FIG. 9a shows a typical situation of a state of the art approach for exchanging data signals between a base station 10 and one or more user equipments 20a and 20b, which are positioned in a known surrounding, like the home. As illustrated, the user equipment 20a and 20b may be a smartphone, tablet PC or notebook. The exchange between the user equipment 20a/20b and the base station 10 is performed by means of a small cell base station 30.
Here, the small cell base station 30, also referred to as an access point, is connected to the base station 10 enabling the connection to the internet background by a plurality of antennas. In detail, base station 10 has three antennas 12a, 12b and 12c, wherein the access point 13 has two antennas 32a and 32b. In such a configuration, the base station 10 and the access point 30 form a 3×2 MIMO system (MIMO: Multiple Input Multiple Output). This has the purpose that 2-fold spatial multiplexing can be used as it is implemented or planned for communication standards like UMTS or LTE. The access point 30 forwards the data to the user equipment 20a and 20b, e.g. by using short range radio communication standards like Wi-Fi. In the shown example, the user equipment 20a and 20b possesses two antennas (not marked by reference numerals), so two 2×2 MIMO systems together with the access point 30 are formed such that again 2-fold spatial multiplexing may be used. Note that such a Wi-Fi system typically uses a different frequency band than the mobile communication system (between 30 and 10).
FIG. 10b shows a simple alternative, wherein no access point is present in the home. Here, the user equipment 20a and 20b are connected directly to the base station 10. Two 3×2 MIMO systems are present due to the fact that the user equipment 20a and 20b possesses at least two antennas, wherein the base station 30 possesses three antennas 12a, 12b and 12c. This enables that 2-fold spatial multiplexing may be used.
Unfortunately, current 4G and Wi-Fi systems cannot reach the high data rates motivated above: The capacity that can be transmitted for every transmit antenna of a base station (or AP in the case of Wi-Fi, respectively) is limited by the used signal constellation, and similarly, the capacity received for every receive antenna is limited by the used signal constellation as well. For instance, using 64-QAM constellations cannot reach higher spectral efficiencies than 6 bit/s/Hz per transmit or receive antenna. Hence, there are two ways to increase the overall data rate of a communications link:
Firstly: Increase the available frequency bandwidth: current systems mostly work in the sub-6 GHz bands (with the exception of some Wi-Fi frequency bands above 6 GHz); frequencies in this range are sought after for various applications and services and are hence scarce. Possibly, further digital dividends can be obtained from the spectrum part that is currently still occupied by TV broadcasting.
Secondly: Increase the number of transmit and receive antennas: possibly, the number of antennas at the base station side can be increased significantly, for instance using distributed antenna systems (DAS). However, for the User Equipment (UE) side, the physical dimensions of the terminal limit the number of antennas that can be integrated. To achieve sufficient decorrelation of the channel coefficients between each transmit and each receive antenna, the spacing between the transmit antennas and also between the receive antennas has to be at least 0.5·λ [gesbert03], where λ is the employed wave length. For 1 GHz carrier frequency, λ is 30 cm and for 6 GHz, it is 5 cm. Therefore, current handheld user equipments carry typically only 2 antennas, and even for tablet or notebook-size user equipments, 4 antennas does almost never provide four times the throughput of one antenna because of the resulting correlation between the antennas. More than 4 antennas are not considered useful for any handheld user equipment device.
As an example, assume that the goal is to transmit to a user equipment at a data rate of 10 Gbit/s (observe that this is a realistic objective assumed in current discussion about the future 5G standard). Let us assume that a future base station can allocate up to 300 MHz (using methods like carrier aggregation) in the sub-6 GHz bands, and that a single user equipment is allocated 50% of the total time-frequency-resources in the downlink. The base station might possess quite many antennas, while the phone-size user equipment is limited to 2 antennas. Hence, only two individual streams can be spatially multiplexed. Each of them has to reach a spectral efficiency of
            10      ⁢                          ⁢      Gbit      ⁢              /            ⁢      s              300      ⁢                          ⁢              MHz        ·        50            ⁢              %        ·        2              =      33.3    ⁢                  ⁢    Bit    ⁢          /        ⁢          symbol      .      Taking into account that a FEC code is needed that adds some redundancy, in this example, each spatial stream would have to employ a signal constellation of at least 234 signal points. It is obvious that such a high constellation cardinality cannot be supported realistically.
Therefore, other solutions are needed for this problem. In the recent years, researchers have started to investigate, what the next-generation mobile communications system 5G could be like. One of the most appealing ideas is to extend the used spectrum to mm-wave, i.e. to the frequency range 30-300 GHz. There are still frequency bands of several hundred MHz or even several GHz, which could be made available to mobile communications. This would be very helpful for providing sufficient bandwidth such that the spectral efficiency does not need to be as high as shown in the example above. However, the range of coverage for the signals at such high frequencies is much smaller than in the sub 6 GHz bands. For instance, the oxygen molecule has its resonance frequency between 57 and 64 GHz. Within this frequency range, oxygen absorbs much of the transmitted power. Building walls are also severe obstacles of mm-wave signals that cause massive attenuation. mm-wave communication quite resembles optical propagation, which hardly allows any communication when the communication link is non-line-of-sight (NLOS).
These arguments are the reason why for 5G mostly the concept of a two-tier network is considered. This concept is in fact similar to what is shown in FIG. 10a, when the AP is replaced by a Small Cell Base Station (SCBS) and the Wi-Fi connections (solid lines) are replaced by mm-wave links. As APs for W-Fi-like systems are very similar to SOBS, both will be represented by the term SOBS in the sequel, while the term base station (BS) represents a macro-cell base station in this document. The name “two-tier network” comes from the fact that in a first tier, the data is exchanged between base station and SOBS, while in a second tier, the data exchange occurs between SOBS and user equipments.
The (wired or microwave link) backhauling for such a system has to connect only the base stations but not the SOBS to the backbone, which ensures a relatively modest cost of the entire system.
Often both ends of the communication link (source and destination) are located within the same small cell (e.g. downloading a video from a server inside the home to the user equipment in the same building), but in other cases, the user necessitates a high data rate to be provided from or to the base station (e.g. downloading the video from the cloud to the user equipment). In this case, similar data rate limits apply for the link between base station and SOBS as shown in the example above. Let us assume that the SOBS possesses 6 antennas for the sub 6 GHz communication to the base station, which is already a large number of antennas for a relatively small device as a SOBS, but which would allow up to 6-fold spatial multiplexing. Coming back to the above example, in order to achieve 10 Gbit/s, each spatial stream has still to achieve a spectral efficiency of approximately 11.1 bit/symbol. That means that at least a 1024-QAM or 4096-QAM has to be used in each spatial stream. Such a large constellation needs a very high SNR to work and is difficult to demodulate correctly (due to imperfect channel estimation, phase noise, transmitter and receiver non-linearities, signal quantization etc.). Moreover, such an SOBS has to be placed in almost every room and has to be quite large in order to separate its 6 antennas sufficiently in space and thereby decorrelate their respective propagation paths. From both points, the communication in the macro-cell between the macro-cell base station and the SOBS represents a kind of bottleneck for the overall communication system.
Therefore, there is the need for an improved approach. An objective of the present invention is to provide an universally applicable communication system enabling based on the current base station resources high data rates while avoiding the above discussed drawbacks and a method for effectively organizing the communication system.